1. Field of the Invention
The present invention relates to a thin film transistor using a boron-doped oxide semiconductor thin film and a method of fabricating the same, and more particularly, to a thin film transistor using a boron-doped oxide semiconductor thin film as a channel layer to remarkably improve electrical characteristics and a method of fabricating the same.
2. Discussion of Related Art
In manufacturing thin film transistors, a ZnO-based material, a non-ZnO-based material and a silicon-based material are mainly used to form a channel layer. These materials will be described in detail.
(1) ZnO-Based Material
Methods of adjusting a ratio of zinc and oxygen (Zn/O) when forming a channel layer using ZnO-based materials have been developed to improve performance of the thin film transistors. For example, the methods may include structural approaches of adjusting a band-gap through substitution of La, Ba, Sr, etc., and methods of adjusting post-processes such as low-temperature annealing and laser annealing.
In addition, the ZnO-based materials have been developed to mitigate deterioration of characteristics due to polycrystalline structure of ZnO. For example, single crystallization is improved through a grain growth method, a low-temperature MBE process, etc., and amorphization is improved using a quaternary material formed of indium, gallium, zinc, and oxygen. However, since the patent related to the quaternary material of indium, gallium, zinc and oxygen has been issued to Hosono, Japan, right of use thereof must be licensed.
Improvement of characteristics through adjustment of concentration of N-type and P-type carriers upon formation of a channel layer using ZnO-based materials is being researched. For example, a single element doping method or a low-cost implantation method is used, and a ternary compound semiconductor, for example, ZnSnO, MgZnO, CdZnO, or the like, is used. There is still much research to be performed on additional substitution to a ternary compound semiconductor.
(2) Non-ZnO-Based Material
Non-ZnO-based materials include an In—Ga—Zn—O type amorphous semiconductor and an individual oxide semiconductor of In2O3 or SnO2. While it is difficult to avoid Hosono's original patent using the In—Ga—Zn—O type amorphous semiconductor, and the individual oxide semiconductor of In2O3 or SnO2 has properties lower than that of ZnO, since there has been relatively little research on the amorphous semiconductor, there is a sufficient margin for improvement of the properties through composition adjustment and doping substitution. While there are non-transparent semiconductors such as CdS, ZnS, ZnSe, etc, as non-ZnO-based materials and their properties can be applied to AMOLED, non-transparent properties cause technical restrictions.
(3) Silicon-Based Material
Devices based on amorphous silicon and polysilicon have been implemented. Since an amorphous silicon transistor has a low mobility of about 1 cm/Vs and a polysilicon transistor has a high mobility of about 100 cm/Vs, there may be irregularity in such devices.
The thin film transistors formed of the ZnO-based material, the non-ZnO-based material and the silicon-based material have the following problems.
In the case of the ZnO-based thin film transistors, characteristics of a thin film may vary sensitively depending on humidity in the air, annealing, and manufacturing processes, which may cause stability problems, and a crystalline channel may cause a problem in device uniformity. Meanwhile, deformation of the device due to current and light may be serious.
The thin film transistors having an IGZO channel, in which indium and gallium oxides are applied to ZnO, have problems in that indium and gallium are being exhausted and their prices are high.
In the case of the silicon-based thin film transistors, in particular, the amorphous silicon has a problem of low mobility, and the polysilicon has a problem in uniformity, which causes a disadvantage in a large-sized panel. In particular, stability of the amorphous silicon transistors depending on current is sensitive.
In general, stability of the oxide thin film transistors depending on the current may be sensitive due to a channel thin film or an interface with a gate insulating layer.
In addition, the thin film transistors using ZnS, ZnSe, CdS, etc., are non-transparent, making it difficult to apply them to transparent electronic devices.
For these reasons, the inventors have researched the thin film transistor and discovered that when the channel layer of the thin film transistor is formed using an oxide semiconductor doped with boron, it is possible to improve electrical characteristics and obtain a thin film transistor stable at a high temperature.